Keep in mind that an application can be decided any time on the basis of evidence provided. If you have applied for ,but not yet received police certificates, attach the evidence under the relevant section (application payment receipt, confirmation that application has been received, application form), so the case officer knows that it is on its way. You can add a brief statement as well, outlining when you expect the missing documentation to arrive.It is possible to submit the application now before we have the relevant documents needed ie police checks and relationship evidence?
I strongly advise you to attach the relationship evidence at the time of application. Even if you add it later, it must relate to the period before the application was lodged, so you may as well provide it all straight up.